fix: broken commutativity in join distribution deduction

Join node deduces the distribution of its output tuple based on
the output tuples of its left and right children. The logic of
deduction was broken: if we swap the children the distribution
changed. So, current commit rewrites join distribution deduction.

Also we refactor some parts of the set_distribution() function.
Its code style is still bad, but now it is not so horrible as
before.